The multifaceted concept of impulsivity implies that different impulsivity aspects, mediated by different neural processes, influence behavior at different levels. The nucleus accumbens (NAc) is a key component of the neural processes regulating impulsivity. In this review, we discuss the findings of lesion studies in animals and functional imaging studies in humans focusing on the role of the NAc in impulsivity. Evidence supports that the extent and pattern of involvement of the NAc, and its subregions, the core and the shell, vary among different facets of impulsivity. Data from imaging studies reviewed in this article suggest the involvement of the ventral striatum/NAc in impulsive choice. Findings of animal studies indicate that lesions of the NAc core subregion facilitated impulsivity in tasks involving intertemporal choice, and promoted a risk-averse, less impulsive, tendency in tasks involving options with probability differences. Modification of neurotransmitter activity, especially of dopamine, which is proposed to underlie the changes observed in functional imaging studies, has been shown to influence afferent input pattern in the NAc and the generation of the behavioral output. Parameters of behavioral tasks reflecting response inhibition function are altered by neurochemical interventions and local electrical stimulation in both the core and the shell subregions. In toto, NAc's pattern of neuronal activity, either genetically determined or acquired, has a critical impact on the interindividual variation in the expression of impulsivity. Nevertheless, the NAc is not the only substrate responsible for impulsivity and it is not involved in each facet of impulsivity to the same extent.
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